engleski

Free-volume studies in aluminosilcate ceramics

Ceramic materials based on zeolites and their amorphous aluminosilicate precursors are of great technological interest due to their thermal, electrical and mechanical properties. Suitability of particulate zeolite as a starting material for ceramic materials depends on the type and structure of zeolite and on the type of extraframework cation present inside zeolite structure, i.e. on the factors that determine mechanism of transformation, temperature of transformation and structure of obtained ceramic phase. Synthesis route is extremely important in preparation and determination of final ceramic product. Applying different routes of preparation will result in ceramic materials with different properties even with the same starting compositions, but same in all cases: more dense structure occures. The shrinkage of atomic structure leads to the more or less complicated pore topology. These pores along with specific vacancy-type defects within grains and intergranular boundaries represent free-volume structure of ceramics. Structural changes during thermal transformation can be followed by different methods and techniques, as FTIR, XRD, etc., and as in the case of our investigations by positron annihilation, method that is sensitive to free-volume distributions in solids. Different sizes of open spaces inside material structure that emerge during thermal transformation of zeolites and their precursors, can be corellated to positron lifetimes, and in that way allow succesful application of PALS for structural investigations. In this work positron lifetime spectra of zeolites are compared to the spectra of ceramic materials obtained by their thermal treatment. In dense ceramic materials long-living components of lifetime spectra are present in lesser extent in comparison to those obtained for zeolites. Trapping sites yielding longest lifetimes in zeolites are missing in denser ceramic structure. Analyses of this results give new insights in zeolite properties, as well as sensitivity limits in free-volume studies in ceramics.

Ceramics; Positron annihilation; Amorphous aluminosilicates

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